What You Will See
How To View It Safely
Why Transits Happen
A planetary transit occurs whenever a planet comes between the the earth and the sun. At that time we see a small black disk crawl across the face of our star. Obviously, only the planets Mercury and Venus can ever do this because those are the only planets with orbits smaller than the earth's, and hence the only planets that can get between us and the sun. Although the outer planets can be seen in the same direction as the sun, they are always on the far side as seen by us. Both Mercury and Venus can appear on the far side, too, but no transit will occur in that case.


When Mercury or Venus passes between us and the sun, we call that event an inferior conjunction. If they are on the far side of the sun (but seen in the same area of the sky), we call that a superior conjunction. Mercury moves one complete revolution around the sun every 88 days. Since we view this from the moving earth, it takes a little extra time (28 days) for Mercury to catch up with us so it can be at inferior conjunction again. The time from one inferior conjunction to the next inferior conjunction is therefore 116 days.

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If we find Mercury at inferior conjunction about every 116 days, you might wonder why we don't see a transit of Mercury every 116 days. The explanation is not too difficult to visualize. As the earth orbits the sun it defines a plane we call the ecliptic. Most of the planets orbit in pretty much the same plane, but Mercury orbits in a plane that is tilted 7 degrees to our orbit. From our point of view, therefore, the great majority of the time Mercury passes above or below the sun when it is at inferior conjunction. Mercury's orbit crosses the ecliptic only twice a year: in early May and in early November. So a transit can only occur if Mercury is at inferior conjunction at those times. The next transit after November 2006 will be May 9, 2016.

The QuickTime animation above (6Mb, be patient!) shows Mercury and its orbit from our viewpoint on earth as we go around the sun. Note that most of the time Mercury would pass above or below the sun. Click here to download Quicktime if you don't have it installed.

Produced with Starry Night by Imaginova.
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